1. Field of the Invention
The present invention relates to electrooptical display devices modulating the light passing through them and more specifically to liquid crystal panels.
2. Discussion of the Background
These panels exhibit promising characteristics for making display screens in avionics, in that they are less bulky than the conventional cathode tube screens and they use less power.
On a liquid crystal panel, an image is displayed using juxtaposed colored, or black, elementary dots. An elementary dot corresponds to the light transmitted to its front face by a liquid crystal cell illuminated on its rear face. From the rear to the front, a liquid crystal cell generally comprises a stack of a polarizer, a first transparent substrate, a thin layer of liquid crystal, a second transparent substrate and an analyser.
These transparent substrates comprise electrodes which are also transparent, to which the application of voltage makes it possible to subject the liquid crystal molecules to an electric field perpendicular to the plane of the cell.
A liquid crystal molecule also exhibits two remarkable characteristics; on one hand, it is capable of modifying the polarization of light passing through it, depending on said molecule""s spatial orientation and, on the other hand, an electric field is capable of changing its orientation.
Thus, the rest state (zero voltage) of and the application of voltage to the liquid crystal layer lead to two different arrangements of the liquid crystal molecules in the layer, defining two states (activated state and unactivated state) of the cell such that, for example, in one state the cell allows light to pass and in the other the cell absorbs it. Depending on its state, the cell makes it possible to display a white dot or a black dot. Gray dots can be produced with intermediate voltages which impose other orientations on the liquid crystal molecules. In addition, the insertion of a color filter into the cell stack makes it possible to present a colored dot. This technology makes it possible to display images in black and white or in color.
However, with a liquid crystal panel, the image perceived depends on the viewing angle at which the panel is observed. A high-quality image perceived by the observer when he is looking along the direction normal to the plane of the panel is distorted when he is looking in a direction inclined with respect to this normal direction. It is this which generally limits the use of a liquid crystal panel to observation directions which depart little from the normal to the panel, i.e. the panel has a restricted viewing angle.
For a display screen which must be legible to an observer whose position is not fixed and/or to several observers placed around the screen, such as, for example, a screen in an airplane pilot""s cabin, the restriction in the viewing angle of the liquid crystal panel is a serious drawback.
For the screen user, the liquid crystal layer exhibits defects in optical behaviour. The defects are especially due to the light contrast between the states of a cell, which contrast causes the particular problem of changing with the observation angle and therefore of disturbing the observation of a panel.
This effect is explained by the natural birefringence of a liquid crystal molecule in which the modification by the molecule of the polarization of the light passing through it depends on the relative orientation between this light and the molecule, the change of observation angle leads to a modification of the polarization of the light received by the analyser and therefore to a modification of the transmission of the cell.
The prior art provides partial corrections of this birefringence in certain situations where it is a problem for various types of liquid crystal cells.
We are more particularly interested in cells comprising a twisted nematic liquid crystal and crossed polarizers (one polarizer, one analyser), located on either side of the liquid crystal layer. The helix rotates the polarized light by about 90 degrees. In their unactivated state, the cells strongly transmit the light received. Their activated state corresponds to considerable absorption of the light; an activated cell observed along its normal has a very low light transmission. The main defect of these cells is, in the activated state, a marked increase in the light transmission for observation inclined with respect to the normal to the cell. A black dot observed perpendicularly to the cell becomes clearer when the observer moves away from the normal to the cell, the contrast between white and black decreases with this movement with respect to the normal. The contrast is the ratio of the transmissions of each state that is the ratio of the cell transmission in the activated state to the cell transmission in the unactivated state.
In the activated state, the birefringence of the liquid crystal molecules is undesirable.
The prior art proposes to correct, however incompletely, this birefringence by the addition of a birefringent compensation film into the cell stack.
A first known correction consists in using a uniaxial birefringent film exhibiting a negative anisotropy of refractive index in the direction perpendicular to the plane of the cell. The film is negative uniaxial with an extraordinary optical axis normal to the cell.
Corrections giving increased satisfaction were then developed. Thus, Patent EP 0,646,829 proposes a birefringent film comprising a support exhibiting the characteristics of the first known correction on which a discotic liquid crystal oriented by rubbing is polymerized. It describes a type of birefringent film comprising two negative uniaxial media, each exhibiting an extraordinary optical axis, one medium being parallel to the normal to the cell, the other medium being inclined with respect to this normal on one hand and with respect to the plane of the cell on the other.
With compensation films, the problem consists in further widening the field of observation of a liquid crystal panel in a plane perpendicular to the panel. In particular, for a panel placed vertically, it is endeavored to improve the horizontal viewing angle ensuring good legibility of the panel for an observer moving horizontally to the right or to the left of the panel.
The invention provides a novel solution which consists in uncrossing the polarizers and the compensation films in order to widen the observation angle.
More specifically, the invention provides a display device with a liquid crystal cell comprising a layer of twisted nematic liquid crystal placed between two polarizers and comprising, between the layer of liquid crystal and each of the polarizers, a structure for compensating for the cell contrast variations according to the observation angle, which structure comprises a layer of a negative uniaxial birefringent material with an extraordinary axis perpendicular to the plane of the cell on which a layer of a negative uniaxial birefringent material with an extraordinary axis inclined with respect to the normal to the plane of the cell is superimposed, the orientation of the compensation structure being parallel and in the opposite direction to the projection of the extraordinary axis inclined to the normal to the plane of the cell, characterized in that the pass directions of the polarizers are separated by an angle of 90+A degrees where A is not zero and the orientations of the two compensation structures are separated by an angle of 90+B degrees where B is not zero.
Compensation of the cell contrast variations according to the observation angle corrects the irksome effects of transmission variations of the birefringent cell according to the observation angle. The compensation corrects the cell birefringence.
The novel solution of the invention consists in arranging the polarizers in a particular relative position which is different to that of the prior art in which the polarizers are crossed perpendicularly in such a way that one polarizer can block the light polarized by the other. The particular position may for example be obtained by uncrossing one pair of polarizers of the prior art. The uncrossing corresponds to a rotation about the normal to the cell of one of the polarizers and to a rotation about the same axis, but in the opposite direction, of the other polarizer. The absolute values of the angles of each of the uncrossing rotations are equal or different.
The uncrossing is done in the direction where the increase in the range of viewing angles is the most favorable. The uncrossing will take place in one direction or in another depending on whether the nematic crystal helix generated by the alignment directions of the molecules on the faces of the liquid crystal layer, are a helix in the anticlockwise or clockwise direction.
The uncrossing is described in order to facilitate understanding of the relative positions in the invention, but the relative arrangement of the elements of the cell may, of course, be obtained directly without physically carrying out rotation operations.
The structure for compensating for the cell contrast variations is preferably in the form of a plane film parallel to the plane of the cell and characterized by an orientation direction in this plane. In the invention, the orientation directions of each of the compensation structures, one on each side of the liquid crystal layer, are no longer perpendicular, they are also uncrossed.
In a preferred embodiment of the invention, the absolute values of the angles of each of the uncrossing rotations are different, one preferably being zero.
The alignment directions of the molecules on the faces of the liquid crystal layer are preferably perpendicular.
The uncrossing of the polarizers, as is described in order to facilitate understanding, is preferably done using polarizers which are crossed with each other and are such that each polarizer is crossed with the alignment direction of the face closest to the liquid crystal layer. These relative orientations between a polarizer and the alignment direction of the closest liquid crystal molecules have the advantage of limiting the contrast inversions.
The invention makes it possible to widen the observation field of a liquid crystal panel. In particular, for a vertical panel, the horizontal viewing angle is widened.
Other characteristics and advantages of the invention will appear on reading the description which follows and which is made with reference to the appended drawings in which: